1. Field of the Invention
The present invention relates to a motor control apparatus for compensating delay when reversing rotation of a motor coupled to a feed axis in a machine tool and an industrial machine and an arm of an industrial robot.
2. Description of the Related Art
A servomotor is coupled to a axis (a machine movable unit), such as a feed axis of a machine tool and an industrial machine and an arm of an industrial robot.
Rotation of the servomotor is converted to straight line motion of a table by a ball screw, and a transmission speed of the servomotor is decelerated by a reduction drive.
Regarding the ball screw or the reduction drive, a difference exists in some cases between a stop position in a positive direction and a stop position in a negative direction with respect to a certain position. Generally, the above-described difference is referred to as a backlash, which causes a decrease in position accuracy.
FIG. 6A to FIG. 6C are drawings illustrating a backlash. In FIG. 6A, a movable unit WA moved by a motor, not illustrated, and a driven unit WB driven by the movable unit WA are illustrated. The movable unit WA includes projections A1 and A2 on both ends and the driven unit WB includes a protrusion B on its center. Thus, for example, when the movable unit WA moves to a right direction, an inner edge of the projection A1 on one end of the movable unit WA engages with one edge of the protrusion B of the driven unit WB. Accordingly, the movable unit WA and the driven unit WB move together to the right direction.
When the motor rotates reversely and the movable unit WA moves from the right direction to a left direction, as illustrated in FIG. 6B, the movable unit WA moves to the left direction. As illustrated in FIG. 6C, when an inner edge of the projection A2 on the other end of the movable unit WA engages with another edge of the protrusion B of the driven unit WB, the movable unit WA and the driven unit WB move together to the left direction.
As described above, when the motor rotates reversely, the movable unit WA needs to move a predetermined moving amount referred to as a backlash before engaging with the driven unit WB. A backlash C illustrated in FIG. 6A and FIG. 6C may cause a decrease in position accuracy. Thus, a compensation amountcompensation amount regarding the backlash C is generated, and the compensation amountcompensation amount is added to a position command of the motor when rotation is reversed.
A machine which performs position control of the driven unit WB based on position information of the motor without acquiring position information of the driven unit WB is a semi-closed controlled machine. The semi-closed controlled machine issues, to the motor, a compensation position command obtained by adding a backlash length to a move command after reversing of a speed command and moves the driven unit WB by an amount of the move command.
A machine which can acquire the position information of the motor and the position information of the driven unit WB both, namely a full-closed controller includes a sensor for the driven unit WB, so that it is only necessary to be provided with a desired value as the move command. In the full-closed controller, the driven unit WB starts moving after the motor has moved by a backlash length when reversing the speed command, and thus delay occurs. Therefore, the full-closed controller may include a speed command compensation function of accelerating the motor after reversing the speed command.
The above-described two techniques perform appropriate compensations at appropriate timings, and compensation amountcompensation amounts and compensation timings are determined in prior to operations. Therefore, these two techniques adopt feedforward control.
On the contrary, according to Japanese Laid-open Patent Publication No. 2014-054001, a position of a motor in a backlash is acquired, and then a backlash compensation amountcompensation amount is determined. In this case, the backlash compensation amountcompensation amount is determined using a current position of the motor in the backlash, so that the technique in Japanese Laid-open Patent Publication No. 2014-054001 adopts feedback control. According to Japanese Laid-open Patent Publication No. 2014-054001, an error between a movable unit and a driven unit when the movable unit engages with the driven unit is regarded as an initial error. A compensation amountcompensation amount is calculated by subtracting an error between the movable unit and the driven unit at current positions from the initial error.
According to Japanese Laid-open Patent Publication No. 2014-054001, a backlash is interpreted a mere gap between tooth faces. However, in normal machines, a backlash is a combination of a mere gap and elastic deformation generated by force acting between the movable unit and the driven unit. In addition, in some machines, there is very little above-described gap, and lost motion based on elastic deformation is dominant. Elastic deformation is generated by torsion of a coupling, especially a resin coupling, coupling the motor and the ball screw and expansion and contraction of the ball screw.
In the case where lost motion based on elastic deformation is caused, when the initial error is calculated in Japanese Laid-open Patent Publication No. 2014-054001, magnitude of a force acting on a backlash end and magnitude of a force necessary for reversing rotation of the motor are different, and there is a possibility that a calculated compensation amountcompensation amount differs from a required compensation amountcompensation amount.
When the compensation amountcompensation amount includes excess or deficiency, shape accuracy of a workpiece is deteriorated. Specifically, if the compensation amountcompensation amount is too small when machining a convex portion of a workpiece, and if the compensation amountcompensation amount is too large when machining a concave portion of the workpiece, a protrusion may be formed on the workpiece. In contrast, if the compensation amountcompensation amount is too large when machining a convex portion of a workpiece, and if the compensation amountcompensation amount is too small when machining a concave portion of the workpiece, a notch may be formed on the workpiece. In particular, when the compensation amountcompensation amount is too large, a notch on a machining surface of the workpiece seems like a scratch, and it is highly likely to be determined as defective machining in the workpiece which is often viewed as a problem.
The present invention is directed to the provision of a motor control apparatus capable of avoiding generation of an excess compensation amountcompensation amount by considering elastic deformation in consideration of the above-described issue.